I was actually suggesting 21 based aircraft, just noting that the 21st doesn't leave in the first wave and waits until 10.25 for its first sector. If you assume 21 based then the schedule does work with no additional aircraft required, including the 20.00 Ibiza and 20.10 Amsterdam. (Just start at the beginning of the day with 21 and subtract 1 every time there is a departure and add 1 for an arrival. Working down the day you'll see that the cumulative total never falls below 0).
There seem to be some slack periods in the day when additional short routes could be added. For example, "aircraft 21" could theoretically leave at 06.15 and return at 09.55 in time to operate the 10.25 to Barcelona. That's a 3 hr 40 min window, so could do a return flight to somewhere with c. 90 min flight time. Of course, that's just pure speculation at this point.